Most common cause is from poor water quality and very rare cases of nitrogen supersaturation, nutritional deficiencies, injury or tumor.
Often popeye can be the first sign of a bacterial infection and soon the symptom of dropsy may show up.

A natural treatment is Epsom salt 1tsp/gal along with 100% daily water changes for 10 days.
Then maintaining quality water with frequent water changes.
The epsom salt can help with symptoms by decreasing edema/swelling and the antibacterial/fungal properties, however, if no improvement within 5-7 days or he starts to get worse you may want to add a broad spectrum antibiotic, I don't use OTC medication so I can't advise on uses.

I've successfully treated pop-eye with Jungle Fungus Clear for 5 days. However your tank is much too small to use any OTC medication. I believe your tank is .5 gallons and most OTC meds are for uses in 10 gallons (I use 1/2 the dose in a 5 gallon hospital tank).